Detection of insulation overheating is of great importance and interest to both manufacturers and users of any equipment which utilizes electrical insulation. It is of particular interest for applications in rotating machinery because of inherent difficulties detecting overheating of insulation combined with the need to know the condition of the insulation in order to interrupt the operation of the machine before catastrophic damage occurs. Due to the harsh environment existing inside rotating machines and difficulties of measuring insulation condition directly, no reliable prior art method exist for non-intrusive monitoring of the bulk insulation, particularly for detection of overheating.
Many prior art electrical rotating machines, such as electric motors, are cooled by forcing air flow or other media (hydrogen, for example) through the motor. If atmospheric air is used as the media the exhaust is usually discharged into the surrounding environment.
As typical electrical insulating materials are heated above normal temperatures, for example, by localized currents due to insulation failure, localized heating results. Under these circumstances large amounts of gases are released. Such releases begin at a relatively low temperature, generally prior to extensive damage to the machine. If these gas releases can be identified using early detection technique it is possible to de-energize the machine before extensive damage, such as extensive conductor melting, occurs.
However, in the absence of the use of early detection techniques, such as the technique disclosed by this patent application, such localized heating frequently results in severe damage to the machine before these currents are detected and interrupted by conventional circuit breakers. Typical damage includes severe melting of electrical conductors.
The preferred embodiment of the invention comprises a pump to sample differentially the cooling gases at the input and exhaust ducts of the machine, sensors to detect the presence in the cooling gases of products produced when insulation overheats, a data system, a control system, means to analyze the data and means to initiate appropriate action when insulation overheating is detected.
Sensors of the type used in the invention are subject to considerable drift. Additionally the ambient environment in which electrical equipment may normally be operated may contain significant amounts of the gases of the type that are released as the insulation overheats. These factors are compensated for in order to detect the actual breakdown of the insulation, as more specifically described below.
Typical electrical insulating material (epoxy, for example) generates a wide variety of gases including carbon monoxide and hydrocarbons as a result of thermal breakdown. These gases are readily detected using commercially available metal oxide semiconductor (MOS) reducing gas sensors. Variations in sensor characteristics, such as variations in sensitivity among sensors to a specific change and changes in sensitivity with time are compensated for to substantially reduce the adverse impact of sensor characteristics on the detection process.
Typical values for gaseous materials produced as insulation thermally decomposes (under a certain set of conditions) are shown in the following chart.
TABLE 1 ______________________________________ Epoxy Insulation Thermal Decomposition Products Temperature .degree.C. 130 150 195 195 225 245 290 ______________________________________ Carbon 630 474 1120 396 916 11690 39260 Monoxide Carbon Dioxide 1340 1260 2410 2480 2480 25820 47300 Methane 18 14 19 7 30 338 1000 Acetylene 0 0 0 0 0 14 50 Ethylene 10 0 17 0 59 533 1030 Ethane 0 0 0 0 0 39 96 Propane 27 14 34 11 37 170 230 Butylene 0 0 0 0 0 0 0 Butane 0 0 0 0 0 0 0 ______________________________________ All values are in microliters/liters (.mu.L/L)
Many of the gases released as insulation thermally decomposes are also present in the environment. Additionally, as discussed above, currently available MOS reducing gas sensors are subject to variations in sensitivity and long term drift. These factors are compensated for in two complementary ways.
First, advantage is taken of the fact that the process of thermal breakdown of insulation is very rapid in rotating machines, resulting in rapid changes in the concentration of breakdown products in the cooling gas. Changes in the concentration of these gases in the environment are typically slow, permitting environmental changes over the detection interval to be ignored. Secondly, a differential technique is used to compare the concentration of products contained in the cooling gas flowing in the exhaust duct of the equipment being monitored to the concentration of products contained in the gas flowing in the input duct to compensate for slow environmental changes in gases which are detected by the sensors (and thus the ambient gases). These two techniques overcome sensor drift, differing sensor sensitivity and changes in the ambient environment.
Selected gaseous products previously discussed, are produced at relatively low temperatures, thus permitting detection of insulation deteriorations before extensive machine damage occurs. This non-intrusive technique was specifically developed for early detection of overheated insulation in the stator, rotor and core insulation systems in a wide variety of air cooled electric motors and electric generators. It can also be used to detect abnormal conditions in any other equipment provided the abnormal condition results in the release of gases which are detectable by the sensor.